The contribution of organic matter to the alkaline reserve of natural waters

The contribution of organic matter (humic compounds) to the alkaline reserve of seawater in the Sea of Japan, in the Razdol’naya River estuarine waters, and in the interstitial waters of the sediments of the Sea of Okhotsk was characterized using two procedures for alkalinity measurements: the method by Bruevich and that of the sample equilibrium with air. It was found that the surface waters of the Sea of Japan contained about 20 μmol/kg of alkalinity of organic origin, and this value twofold decreased with depth. For most of the actual cases of the calculations of the seawater carbonate system, this value may be neglected. Meanwhile, the contribution of organic alkalinity to the Razdol’naya River waters amounts to nearly 120 μmol/kg. It was shown that, if this value in the calculation of the carbonate system of the Razdol’naya River estuary-Amur Bay is neglected, this may cause gross errors in the values of the partial pressure of carbon dioxide (the error might be over 1500 μatm) and in the dissolved inorganic carbon (an error over 150 μmol/kg). The maximum absolute contribution of the humic matter (over 300 μmol/kg) was found for the interstitial waters in selected sediments of the Sea of Okhotsk. In the interstitial waters of these sediments, humic matter concentrations as high as 300 mg/l were detected. The data obtained show that the determination of the amount of humic matter must be an indispensable condition for an adequate analysis of estuarine carbonate systems and of the interstitial water in reduced marine sediments.